1. Technical Field
This invention relates to the field of artificial lighting and more specifically to luminaires of the direct-indirect type for illuminating interiors.
2. Background
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
In the field of artificial lighting, and particularly in applications for illuminating interiors, the luminaire structure surrounding a source of light directs light to one or more surfaces of an interior space, or additionally, to one or more objects it contains. A portion of the light from the light source can fall directly on one or more surfaces of that interior space or on one or more objects it contains without first impinging on or passing through any luminaire structure. Light sources used in luminaires include lamps such as linear or circular tubular fluorescent lamps, incandescent light bulbs, light emitting diodes (LEDs), and high intensity discharge (HID) lamps such as ceramic metal halide (CMH) lamps.
Typically, a luminaire can be a light fixture that projects light on the ceiling of a room (an up-light fixture), the floor of a room (a down-light fixture), one or more walls of a room, one or more objects in a room, or any combination of the above.
Attributes used to evaluate the performance of a luminaire may include any one or more of the following:    Efficiency, which is the number of lumens output flux per electrical watt of input power.    The illuminance generated.    The incident down-light distribution, which determines the illuminance uniformity it produces on the floor of a room from both direct and indirect down-light flux. “Direct” down-light flux falls on the room's floor (or on objects standing on that floor) directly from the luminaire. “Indirect” down-light flux falls on a room's floor (or on objects standing on that floor) after first reflecting from the room's ceiling or walls.    The angular distribution of direct down-light intensity. For example, a “bat-wing” intensity angular distribution may be preferred, which can make illuminance from direct down-light flux more uniform.    The ratio of down-light flux to up-light flux. Luminaires comprising a mix of down-light and up-light are described in U.S. Pat. No. 4,472,767, U.S. Pat. No. 5,884,994, U.S. Pat. No. 6,457,844, and WO 03/036161 A1.    The glare it generates, which is most important when illuminating work areas typically used for tasks performed during many hours of a workday. For example, it is important to limit glare in areas where extensive use of computer monitors exists.    Aesthetic appeal. For example, crystal chandeliers are often integrated into the design of a light fixture to enhance its beauty. Accordingly, a “crystalline effect” is often sought in luminaire designs.    Costs to fabricate, operate, and maintain.    Generally, a luminaire's performance is best when its efficiency is high. High efficiency lowers lighting costs owing to fewer watts of electrical power needed to generate a required luminous output and fewer luminaires needed to light a given room to desired levels of illuminance and illuminance uniformity. A luminaire's performance is also best when the illuminance it generates on the floor of a room is uniform to a specified degree, the glare it produces is sufficiently low, and it is aesthetically pleasing. It is also desirable for the costs associated with the luminaire's fabrication, operation, and maintenance are low.
Specific values or limits for each of these attributes depend on the luminaire's application and on the end user's preferences. Two lighting standards often used for evaluating interior lighting are DIN 5035 parts 1 and 7, and ANSI/IESNA RP-1-04, both incorporated by reference herein in their entirety.
Incorporated herein by reference in their entirety are U.S. patent application Ser. No. 10/366,337, filed on Feb. 14, 2003 and U.S. provisional application No. 60/409,269, filed Sep. 10, 2002. Both of these are assigned to the assignee of the present invention.
Also, incorporated herein by reference in its entirety is U.S. Pat. No. 4,641,315, “Modified Involute Flashlamp Reflector”, granted on Feb. 3, 1987 and assigned to The Boeing Company. This patent discloses a set of parametric equations that can be used to define the shape of cusp reflectors that project light emitted by tubular cylindrical lamps without directing any reflected light back to the cylindrical surface of lamp envelopes. Avoiding back-reflections to the lamp reduces light absorption by the lamp. Accordingly, this improves efficiency by increasing the amount of light flux projected out from a cusp reflector/lamp fixture for a given electrical power input.
Ideally, a luminare    Would have a luminaire efficiency (ratio of the total light output from the luminaires to the total output from the bare lamp(s) that fits into it) exceeding 90%. This lowers operational costs. Also, by requiring fewer luminaires to light a given area, it lowers fabrication and maintenance costs while further reducing operational costs.    Would be a light fixture providing both up-light (indirect) and down-light (direct) illumination or side-light (indirect) and down-light illumination (direct). A combination of indirect and direct illumination can be both highly efficient and aesthetically pleasing.    Would have little or no down-light flux projected at angle of 45 degrees or greater from vertical.    May have down-light flux projected with a “bat-wing” intensity angular distribution. This enhances illuminance uniformity.    Would have a tunable design capable of satisfying user preferences by providing down-light to up-light or down-light to side-light ratios over a wide range of values (up to unity, or higher) while maintaining high luminaire efficiency.